The present invention relates to an improvement in a contactless ignition system having a dwell time control circuit for an engine such as disclosed in Japanese patent publication, Sho 41-2803, published on Feb. 22, 1966, assigned to the same assignee of the present application.
In the above Japanese patent publication, there is shown an ignition control system having a dwell time control circuit which comprises an AC generator for generating an alternating current signal having timed relationship with the engine. Such signal is shown in (a) of FIG. 1 of the present application in which an engine speed responsive bias voltage changing circuit for generating speed responsive DC signal, as designated by V.sub.O (low speed) and V.sub.O ' (high speed) in (a) of FIG. 1 of the present application, a waveshaping circuit for generating a rectangular control signal and a switching circuit for controlling charge and discharge of an ignition coil in response to the rectangular control signal. The waveshaping circuit is so arranged that the width of the rectangular signal thereof is made longer in accordance with the engine speed responsive DC voltage as the engine speed increases, as shown in (b) and (c) of FIG. 1 of the present application. As a result, the portion of each alternating cycle during which the ignition coil is energized is increased as the engine speed increases, thereby ensuring a good igniteability of the engine even in the high speed range-thereof.
However, it has been found to be a drawback that the ignition timing at which the discharge of the ignition energy is made is changed with the change of the speed responsive DC voltage as designated by T in (c) of FIG. 1 of the present application. This time difference T varies with variation in the wave shape of the AC voltage due to the type, construction, and other characteristics of the AC generator. The difference T also depends on variations in the dwell time requirement and in manufacturing errors of the igniton system. Generally, the ignition timing is made advanced in a high speed range of the engine by spark advance mechanisms such as a centrifugal advancer and a vacuum advancer. However, it is practically difficult to accurately compensate such time difference T by those mechanisms.